This invention relates to an acidic fluid that increases in viscosity as it is pumped into a subterranean formation. More particularly, it relates to a fluid system—that contains a concentration of a viscoelastic surfactant that is initially insufficient to cause the fluid to gel—that gels as the fluid flows through a permeable subterranean formation. Most particularly it relates to a method of treating a subterranean formation with such a fluid system.
In many oilfield operations there are occasions when it is desirable to pump a low viscosity fluid that becomes more viscous after it has been injected into a borehole or into a formation. Initially, low viscosity might be desirable, for example, to reduce the hydraulic horsepower needed for pumping, to facilitate mixing procedures at the surface, or to decrease the resistance to flow of the fluid through the pores of a subterranean formation. Subsequently, higher viscosity might be desired for example to reduce the flow rate of the fluid through a porous formation or through fractured or lost-circulation zones, for example so that the fluid acts as a diverter. Currently, there are several types of acid systems that are designed to increase in viscosity, but the viscosity increases are dependent upon consumption of some or all of the acid. Some of these systems are polymer-based; polymers may damage formations. Other systems are based on viscoelastic surfactants (VES's) that are much cleaner; however, the existing VES systems require that the initial fluid contain sufficient surfactant to form a gel, and surfactants may be expensive. A fluid that is based on a low surfactant concentration and does not require acid consumption to gel would be desirable.